Exchange Server 2007 administrators often disagree about whether to configure disks on servers as basic disks or dynamic disks. Microsoft provides some recommendations, but which type you choose for your partition structure can vary. In this tip, Exchange expert Brien Posey explains the benefits -- and limitations -- of using basic disks, dynamic disks, master boot records and GUID partition tables.

Overall, I recommend you use basic disks if possible. And Microsoft agrees that this is the best choice for all server roles. Additionally, single copy clusters don't support dynamic disks without the use of third-party disk management software.

Dynamic disks should be used to create a RAID array at the software level. Generally, it's better to use a hardware-based disk array, but if this is out of the question, using dynamic disks to create an array at the Windows level may be your only option.

Creating MBR and GPT partitions

Exchange Server 2007 must be installed on a 64-bit operating system. You can create two different types of partitions on 64-bit editions of Windows Server -- a master boot record (MBR) partition or a GUID partition table (GPT). Although Exchange Server 2007 supports both types, each has tradeoffs and limitations.

MBR partitions are best suited for general use. Microsoft states that using MBR partitions for Exchange Server storage is a best practice.

However, GPT partitions have main two advantages over MBR partitions. First, unlike MBR partitions, GPT partitions are not subject to the 2 TB size limit. This is an important consideration for large organizations with rapidly growing Exchange databases -- though the guidelines for maximum database sizes are far below 2 TB. Microsoft also discourages that organizations place multiple databases on a single partition.

Another advantage of using a GPT partition is that the partition table is replicated and Windows uses a cyclic redundancy check to prevent errors. This means that a GPT partition is slightly more reliable than an MBR partition.

But GPT partitions aren't without fault. Their biggest limitation is that can neither boot off of a GPT partition, nor can you use GPT for the system partition. Since you'd never store Exchange databases or transaction logs on the boot partition or the system partition, these limitations aren't applicable.

Additionally, GPT partitions aren't supported for use on shared disks in Windows clusters. Therefore, if you're planning to configure a mailbox server as part of a single copy cluster, don't use a GPT partition.

Both MBR and GPT partitions support the creation of additional partitions. In fact, GPT supports up to 128 primary partitions. However, Microsoft's best practices state that you should never have more than one partition per LUN.

For example, suppose that you had a large RAID array that was configured as a single LUN and you wanted to divide the array into multiple partitions. Using one partition for an Exchange database and another partition for the transaction logs isn't recommended. In this case, a disk failure could cause you to lose both the database and the transaction logs. Creating multiple partitions, each of which can accommodate a single database, also isn't recommended because the disks would be required to handle multiple I/O streams -- one for each database.

About the author: Brien M. Posey, MCSE, is a five-time recipient of Microsoft's Most Valuable Professional (MVP) award for his work with Exchange Server, Windows Server, Internet Information Services (IIS), and File Systems and Storage. Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal website at www.brienposey.com.

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